Lithium ion secondary batteries are broadly used in small-size information devices such as cell phones and laptop computers because of higher energy density compared to those secondary batteries such as nickel-cadmium batteries and nickel-hydrogen batteries and availability at a high potential. Further, in recent years, since the size and weight reduction of lithium ion secondary batteries is easily achieved, demands therefor have arisen as secondary batteries for hybrid cars and electric cars. Research and development has been carried out for all-solid-state lithium ion secondary batteries using no inflammable electrolyte solution in consideration of the safety. Solid electrolytes to be used for all-solid-state lithium ion secondary batteries are demanded to have a high ionic conductivity.
It has been reported that materials having a cubic garnet-related structure have a high ionic conductivity (see, for example, Patent Literature 1), and research and development of materials having this structure is being advanced. In particularly, a material having a chemical composition represented by Li7-xLa3Zr2-xTaxO12 is reported to have a high ionic conductivity nearly at x=0.5. Since the grain boundary resistance and the interface resistance need to be reduced to the utmost in order to realize a high ionic conductivity, a solid material being a formed body having a high density is desirable. Since a solid material being a formed body having a high density is capable of preventing short circuit between positive and negative electrodes in the charge and discharge process and can be turned into a thin film piece, the solid material gives a possibility to the future size reduction of all-solid-state lithium ion secondary batteries. It is known, however, that these materials having a cubic garnet-related structure are sintering-resistant and the fabrication of high-density formed bodies is difficult.